The invention relates to a high-pressure discharge lamp comprising a discharge vessel which is enclosed, with clearance, by an outer bulb which is provided, at an end, with a lamp cap, which lamp also comprises an ignition circuit including a series arrangement of a glow starter switch and an ohmic impedance.
A lamp of the type mentioned in the opening paragraph is disclosed in GB 1596161. An advantageous aspect of a lamp having a built-in ignition circuit resides in that, in principle, said lamp can suitably be used as a retrofit lamp for an apparatus intended for a high-pressure mercury discharge lamp. State-of-the-art high-pressure discharge lamps, such as high-pressure sodium lamps and metal halide lamps, have a very high ignition voltage and, consequently, the ignition circuit must generate a high ignition pulse, for example of at least 2.5 kV. As a result, comparatively large currents occur in the ignition circuit, which currents are limited as much as possible by means of the ohmic impedance. Consequently, however, the ohmic impedance must also be comparatively large, and a substantial degree of dissipation takes place in the ohmic impedance. This is disadvantageous and requires the use of special high-power ohmic impedances, which has a cost-increasing effect and hence is disadvantageous.
It is an object of the invention to provide a means for counteracting said drawbacks. To achieve this, a lamp of the type mentioned in the opening paragraph is characterized, as a lamp in accordance with the invention, in that the ohmic impedance is formed by a halogen incandescent lamp. As an incandescent element, for example in the form of an incandescent filament, of an incandescent lamp is intended to adopt a high temperature and said incandescent element is also incorporated in a halogen-filled bulb, the incandescent lamp is resistant to comparatively prolonged and comparatively substantial dissipation of the incandescent body. Large-scale use of halogen incandescent lamps in a large power range for illumination purposes further has the advantage that there is ample supply of such lamps, so that a person skilled in the art can readily select, at comparatively low costs, a suitable halogen incandescent lamp for use in the ignition circuit of the lamp in accordance with the invention. Surprisingly, it has been found that the halogen incandescent lamp can also serve as a UV-enhancer. A UV-enhancer has a very favorable effect on the speed and reliability with which the high-pressure discharge lamp is ignited. This is favorable for, in particular, metal halide lamps, as their ignition behavior is influenced to a substantial degree by the presence of free electrons. Preferably, such a metal halide lamp is provided with a discharge vessel having a ceramic wall. The lamp thus formed has the advantage that it combines a favorable ignition behavior with very good light-technical properties, such as a high luminous flux and stable color properties.
Preferably, the series arrangement also comprises a bimetal switch, which is closed in the cold state. This has the advantage that the series arrangement can be electrically switched off as soon as the lamp has been ignited, for example under the influence of the heat development resulting from the discharge. A further advantage resides in that, after lamp extinction and re-application of a supply voltage, ignition voltage peaks will not be generated until after the bimetal switch has cooled to such temperature that it is in the closed state. It is thus precluded that the ignition circuit is needlessly loaded.